Recently, a vertical cavity surface-emitting laser including a semiconductor multilayer reflector (Distributed Bragg Reflector, DBR) is used in a parallel information processing system, a parallel optical communication system, etc.
A conventional semiconductor multilayer reflector (DBR) is composed of two kinds of semiconductor materials each having a different refractive index from the other to increase a reflection factor. That is, quarter-wavelength high and low refractive index layers are piled up alternately to provide the semiconductor multilayer reflector (DBR).
According to the conventional semiconductor multilayer reflector (DBR), energy band is not continuous in heterointerfaces, because energy bandgap is largely different between the high refractive index layer and the low refractive index layer, so that large potential barriers (spikes) occur on a conduction band in the heterointerfaces. Therefore, carrier flowing in the device is impeded by the potential barriers, so that serial resistance is increased.
Accordingly, some means for overcoming the above disadvantage are proposed. That is, the impurity is doped in the multilayer reflector to increase tunnel current by which the potential barriers is decreased. However, not only crystal is deteriorated by the high impurity doping, but also free carrier absorption increases. On the other hand, it is proposed that thin films of intermediate constitution are inserted in the heterointerfaces, and that graded layers are inserted in the heterointerfaces to decrease the potential barriers as described on pages 2496 to 2498 of "Appl Phys Lett 56 (25), Jun. 18, 1990". However, when the graded layers are formed, growing temperature is required to change in short cycle, so that the fabrication is complicated and the yield is decreased.